One or more embodiments of the present invention relate to a casing plug. One or more embodiments of the present invention also relate to a well anchor, which may be used together with the casing plug, but which may also be used with other well tools.
There are different types of well plugs used in hydrocarbon producing wells. Such plugs may be retrievable plugs, i.e. they may be retrieved from the well after their use, or they may be permanent plugs, i.e. they are set permanently and must be milled/drilled into pieces in order to be removed.
The well plug may comprise an anchor device, which in the set state (radially expanded state) is in contact with the inner surface of the well pipe. Its primary object is to prevent upwardly and/or downwardly directed movement of the plug in relation to the well pipe.
The well plug may also comprise a sealing device, which in the set state (radially expanded state) also is in contact with the inner surface of the well pipe. Its primary object is to prevent fluid to pass the annular space between the outer surface of the plug and the inner surface of the well pipe.
Plugs are set by means of a running tool lowered into the well. The running tool is connected to the plug, and at the desired depth, the running tool is actuated and the plug is brought from its run state (radially retracted state) to its set state (radially expanded state).
One common connection interface between a plug and a running tool comprises an inner mandrel of the plug connected to an inner mandrel of the running tool and an outer housing of the plug connected to an outer housing of the running tool. By relative axial movement between the outer housing and the inner mandrel, the plug is brought from its run state to its set state. In order to initiate this relative movement, an axial force larger than a certain threshold is applied to the inner mandrel while holding the outer housing stationary (or vice versa). At this force threshold, a shear stud is sheared off, and consequently relative axial movement is allowed. The shear stud may be located in the plug or in the running tool.
Casing plugs are one type of well plug used during completion of a hydrocarbon well, during temporary plugging and abandonment (P&A) of the well etc. The casing plug is set in the casing pipe by using drill pipe to run the plug, to set the plug and also to retrieve the plug. One or more embodiments of the present invention may provide a casing plug with the following capabilities:                it should be possible to hang off weight under the plug such as drill pipe, bottom hole assembly, sensors, etc.        it should be possible to pump fluid through the plug before an equalizing valve is closed, in order to check the pressure under the plug, for example to check that the completion operation was successful.        the plug should be resettable, e.g. it should be possible to run the plug to a desired position, then set the plug and perform a pressure test, then to run the plug to a new desired position, set the plug again and then perform a pressure test again.        it should be possible to abandon the plug in a set and closed state, i.e. to retrieve the running tool and drill pipe after the setting and closing of the plug.        
Such a resettable casing plug is difficult to achieve with shear studs, hence, shear studs for the resetting configuration should be avoided.
Typically, such setting and resetting of the plug have been actuated by rotation of the drill pipe. A disadvantage is that it is difficult to ascertain how much the lower part of the drill string has rotated in relation to how much the upper part of the drill string has rotated, particularly for long drill strings. Another disadvantage is that there is a risk that one of the joints of drill pipe will be unscrewed, instead of bringing the plug to the desired state.
Consequently, one or more embodiments of the present invention may achieve a casing plug which has the above capabilities while avoiding the disadvantages of the rotating drill pipe.
Another known way of initiating the setting operation of the plug has been to use so-called drag blocks to create friction between the plug and the inner surface of the casing. Such drag-blocks are typically connected to the plug via coil springs, allowing the drag-blocks to move in relation to the plug due to irregularities of the inner surface of the casing etc. The friction is however sufficient to form an initial anchor which keeps some parts of the plug stationary while moving other parts by means of the pipe string. One example is shown in U.S. Pat. No. 3,714,983.
One known way of achieving fluid actuated plugs is to provide the plug with a closed compartment at the surface. When the plug is lowered into the well, the pressure of the fluid in the annulus outside the plug is typically much higher than the pressure within the closed compartment. Hence, by opening a passage between the annulus and the compartment, fluid will flow from the annulus and into the compartment—a fluid flow which may be used to bring at least parts of the plug from the run state to the set state. An initial operation is here always needed to open the passage at the desired location in the well. One example is shown in U.S. Pat. No. 3,294,171. Here, the opening of the passage is initiated by detent means which are moved upwards into engagement with a joint or other obstruction provided in the inner surface of the casing itself. Moreover, this approach may also requires shear pins.
Hence, in the above two approaches, a first, initial contact between the plug and the casing is needed in order to achieve a second contact in the form of a proper anchoring of the plug to the casing. Moreover, the two approaches above are irreversible (opening of the passage to the atmospheric compartment and the breaking of shear pins).
One or more embodiments of the invention may provide an improved initial anchoring of the casing plug to the casing—without the use of drag blocks and/or gas filled compartment of the above prior art.